> MacOS was absolutely derived from BSD through NeXTSTEP.

The OS-X (now branded as "macOS") kernel was not, and is not, a derivative of the FreeBSD kernel, or any other BSD, even though macOS/OS-X has a FreeBSD kernel component due to its Mach heritage. The userland tools are however BSD. OS-X's kernel is XNU and from the XNU GitHub repo[0]:

  XNU kernel is part of the Darwin operating system for use 
  in macOS and iOS operating systems. XNU is an acronym for X 
  is Not Unix. XNU is a hybrid kernel combining the Mach 
  kernel developed at Carnegie Mellon University with 
  components from FreeBSD and a C++ API for writing drivers 
  called IOKit. XNU runs on x86_64 and ARM64 for both single 
  processor and multi-processor configurations.

EDIT:

In theory, XNU could simultaneously run the existing FreeBSD subsystem alongside Linux and/or MS-Windows ones. In practice, this would be a herculean effort fraught with difficulty.

See QNX[2] for another example of a micro-kernel OS architecture.

0 - https://github.com/apple-oss-distributions/xnu

1 - https://books.apple.com/us/book/mac-os-x-internals/id4343583...

2 - https://en.wikipedia.org/wiki/QNX

Large parts have been rewritten: they very different and don't show any BSD heritage.